1. Field of the Invention
The present invention relates generally to controlling radiant energy deposition in imaging devices which employ moving image receiving surfaces upon which latent images are formed by charging and/or discharging these image receiving surfaces with radiant energy, and more particularly to controlling the intensity of a stream of radiant energy such as, for example, light or ions, which is directed toward the imaging surface to form the latent image so that portions of the image receiving surface are uniformly charged regardless of fluctuations in the velocity at which the image receiving surface is moved relative to a source of the radiant energy (also known as velocity error).
2. Discussion of Related Art
A large number of image forming, or imaging, devices are available which produce a latent image on an image receiving, or imaging, surface by directing a stream of radiant energy, which is appropriately modulated based on the image to be formed, toward the imaging surface. The latent image is then made visible on a copy sheet by suitable development apparatus to form a permanent image on the copy sheet. For example, the imaging surface can be a photoreceptive belt or drum which is uniformly charged and then moved past a stream of light, the intensity of which is imagewise modulated, to form the latent image thereon. The light can be modulated, for example, by reflection off an original document (which contains dark and light portions based on the image contained thereon), or by providing a plurality of light sources (such as LED's) which are individually addressed with data representing an image so that they selectively emit or do not emit light towards the photoreceptive surface. In either case, the light which reaches portions of the photoreceptive surface causes the charge on these portions to dissipate to a varying degree based upon the intensity of the light and the duration of its exposure onto that portion of the photoreceptive surface. Similarly, an ionographic imaging device directs a modulated stream of ions towards a moving electroreceptive surface (such as, for example, a drum or belt) to selectively charge portions of the electroreceptive surface in imagewise fashion. The darkness of the images formed on the copy sheet depends on the density of the ion stream and the duration of its exposure onto portions of the electroreceptive surface receiving that image.
Thus, in imaging devices which direct a stream of radiant energy toward a moving imaging surface, the amount of charge which is applied to, or removed from, the imaging surface is related to the speed at which the imaging surface is moved. While it is preferred to maintain the speed of the imaging surface at a constant set speed, and while over time the speed of the imaging surface can, in general, be maintained constant, the actual speed tends to fluctuate and thus, at any instantaneous moment, varies from the set speed. These speed fluctuations can cause inconsistencies in the line-by-line quality, size and darkness of the formed images. These speed fluctuations can be caused by, for example, fluctuations in the input power to the imaging device as well as "gear chatter" in the drive train which moves the imaging surface. Even in systems which selectively operate at more than one set speed, and thus take account of the different possible selected speeds, fluctuations in the actual speed of the imaging surface from the selected set speed results in the same type of inconsistencies in the print quality, etc.
Imaging systems have been introduced which take account of fluctuations in imaging surface speed to partially compensate for the inconsistencies caused thereby. For example, it is known to monitor the position of the imaging surface (e.g. by using a rotary motion encoder) and to control the output of data by an image bar (which forms the latent image on the imaging surface) so that characters are formed at the proper locations on the imaging surface. This process is also known as "reflex printing" and assists in forming characters at the proper locations on the imaging surface. See, for example, U.S. Pat. Nos. 4,575,739 to De Schamphelaere et al, 4,839,671 to Theodoulou, et al and U.S. Pat. No. 5,081,476 to Genovese, which is assigned to the same assignee as the present application.
While the above systems ensure that each new line of information is started at the correct time, any velocity error in the imaging surface velocity will cause a variation in delivered radiant energy (e.g., light intensity to a photoreceptor or ion density to an electroreceptor) that may result in a perceptible error. That is, since the above systems maintain the intensity of the stream of radiant energy which is directed to the imaging surface constant, the stream of energy will be exposed to the imaging surface for different amounts of time depending on the instantaneous speed of the imaging surface, causing different amounts of charging or discharging of the imaging surface to occur at these different speeds. Perceptible differences, or error, in the darkness of the characters in the resulting image has been shown to be particularly evident in low-to-medium density images made with continuous-tone ionography. The same problem would occur in many photon driven continuous-tone methods.
While the above referenced U.S. Pat. No. 5,081,476 to Genovese recognizes that "reflex printing" alone does not fully compensate for all of the inconsistencies caused by speed fluctuations, that application does not fully eliminate all of the inconsistencies mentioned above. The Genovese application controls the time period during which a stream of ions is permitted to flow towards the imaging surface at each line of information so that this time period is constant at each line of information regardless of any fluctuations in the speed of the imaging surface. Gating electrodes are used to selectively permit or block the flow of a modulated stream of ions towards an imaging surface. Thus, the device disclosed by Genovese ensures that substantially equal amounts of energy are applied to the imaging surface at each line of information. However, since the velocity of the imaging surface may be different at each of these lines of information (due to the above-described velocity fluctuations), different amounts of charge are applied to each unit area of the imaging surface and, thus, the resulting darkness of the final output image is not uniform.
U.S. Pat. No. 3,496,351 to Cunningham, Jr. discloses a corona control circuit for controlling the charging of a photoreceptive drum. Images are formed on the drum by rotating the drum in stepwise fashion and applying a light image, one line at a time, to the drum each time the drum is stopped. Cunningham, Jr. recognizes that the duration of the stopped time (known as the dwell time) decreases at higher imaging speeds and that the charging of the drum must be controlled in relation to the dwell time so that the drum is uniformly charged. Accordingly, the intensity of the charge applied to the drum is increased at shorter dwell times and decreased at longer dwell times based on the selected imaging speed. However, Cunningham, Jr. does not compensate for velocity fluctuations from a set speed (i.e., the selected imaging speed) and thus does not measure or compensate for differences between the actual speed of an imaging surface and a set speed. The intensity of the energy stream which exposes the imaging surface to intelligible, imagewise information is also not controlled.
U.S. Pat. No. 4,431,302 to Weber discloses a system which compensates for differences between the actual current flow and the desired current flow from a corona charging electrode which charges an electrochargeable medium by varying the speed of the electrochargeable medium. As with Cunningham, Jr., Weber recognizes the general relationship between the intensity of an energy stream and the speed of an imaging surface which is charged by this energy stream but Weber does not teach or suggest the present invention. In fact, Weber apparently assumes that the imaging surface speed can be precisely controlled or that fluctuations in the imaging surface speed are inconsequential.
U.S. Pat. Nos. 3,935,517 to O,Brien and 4,480,909 to Tsuchiya also disclose the general relationship between energy stream intensity and imaging surface velocity required to achieve uniform charging of the imaging surface, but do not teach or suggest the present invention. All the patents and patent applications cited are incorporated by reference herein.
Xerox Corp. U.S. Pat. Nos. 4,584,592 to Tuan et al, 4,646,163 to Tuan et al, 4,524,371 to Sheridon et al., 4,463,363 to Gundlach et al., 4,538,163 to Sheridon, 4,644,373 to Sheridan et al., and 4,737,805 to Weisfield et al. disclose typical ionographic imaging devices including ionographic head construction, modulation circuitry, and ionographic device architecture, and are herein incorporated by reference.